The exemplary embodiments relate to a projector.
FIG. 12A is an explanatory diagram of an optical system of a related-art projector 1000a that includes a micromirror modulator. FIG. 12B is an explanatory diagram of a light source 110a of the related-art projector 1000a that includes a micromirror modulator.
As shown in FIG. 12A, the related-art projector 1000a that includes a micromirror modulator, includes an illumination system 100a. The illumination system 100a has a light source 110a that emits converging light, and an integrator rod 120a that converts the light from the light source 110a to light having a more even intensity distribution. The projector 1000a also has a relay optical system 140a that guides the light from the illumination system 100a to a region being illuminated, a micromirror modulator 200a that modulates the light from the relay optical system 140a in accordance with image information, and a projection optical system 300a that projects the light modulated by the micromirror modulator 200a. The projector 1000a further has a color wheel 130a on the light incident side of the integrator rod 120a. 
As shown in FIG. 12B, the light source 110a includes an ellipsoidal reflector 114a, an arc tube 112a having a luminescence center in the vicinity of a first focus of the ellipsoidal reflector 114a, and a reflection system 160. The reflection system 160 allows the light in the center of the converging light exiting from the ellipsoidal reflector 114a to pass through, and reflects light in the periphery back to the direction opposite to the direction of incidence.
Thus, with the related-art projector 1000a that includes a micromirror modulator, the angle of the converging light emitted from the light source 110a can be decreased by the operation of the reflection system 160. Accordingly, the angle of incidence of the converging light that enters the integrator rod 120a can be decreased, allowing the light utilization efficiency of the illumination system to easily be increased (for example, refer to JP-A-2003-222820).
However, in the related-art projector 1000a that includes a micromirror modulator, the reflection system 160 must have an equal size to that of the ellipsoidal reflector 114a since it needs to reflect the peripheral light of the converging light exiting from the ellipsoidal reflector 114a. The related art projector 1000a thus poses the problem of an increasing scale of the light source. This results in decreasing the efficiency of cooling the light source because the large-scale reflection system 160 closes the opening of the ellipsoidal reflector 114a. 
Accordingly, in the related-art projector 1000a that includes a micromirror modulator, a reflection system 170, as shown in FIG. 13, can be used in place of the reflection system 160 to simplify the structure of the light source and to prevent a decrease in light-source cooling efficiency. FIG. 13 shows another related-art light source 110b. 
As shown in FIG. 13, the related-art light source 110b includes an ellipsoidal reflector 114b, an arc tube 112b having a luminescence center in the vicinity of a first focus of the ellipsoidal reflector 114b, and the reflection system 170 that reflects the light radiated from the ellipsoidal reflector 114b to the region being illuminated back to the ellipsoidal reflector 114b. The reflection system 170 is made of a reflection film formed on the outer surface of the bulb of the arc tube 112b. 
Thus, with the related-art light source 110b, the reflection system 170 can reflect the light radiated from the ellipsoidal reflector 114b toward a region being illuminated with the reflection film formed on the outer surface of the bulb of the arc tube 112b, simplifying the light source. This also prevents or discourages a decrease in the light-source cooling efficiency (for example, refer to International Publication No. 03-033959).
However, an analysis of the inventor shows that arc tubes that are in circulation at present vary to a non-negligible extent in shape and size of the bulb, the position of the electrodes, and the position of the luminescence center from one arc tube to another. Accordingly, the use of the reflection system 170 for the light source 110b of the projector 1000a having a micromirror modulator, in place of the reflection system 160, ensures simplification of the light source and prevents or discourages a decrease in the light-source cooling efficiency. On the other hand, the light reflected by the reflection system 170 toward the ellipsoidal reflector does not necessarily pass through the illuminator of the arc tube. As a result, part of the light reflected by the ellipsoidal reflector 114b does not reach the plane of incidence of the integrator rod 120a, decreasing light utilization efficiency and increasing stray light.